[Template-haskell] How to get list of instances of a given class?

Dimitry Golubovsky golubovsky at gmail.com
Fri Dec 16 10:02:13 EST 2005


Thanks for your reply. This saves me a lot of time as I would not
spend it just to discover this was impossible anyway.

Is it possible to publish a list of what is possible to
access/introspect, and what is not possible? In 6.2.2? In 6.4.1?

For instance, on Dec 12, you explained how module imports may be
accessed, so it is already possible [1], at least in 6.4.1

On 12/16/05, Simon Peyton-Jones <simonpj at microsoft.com> wrote:
> There's currently no way to get the list of instances of a class.  It
> would not be very difficult to add this.   Any volunteers?  Or I could

> do it if there is serious demand.  How much difference would it make?

As for myself, I may get around by writing some sort of plugins to
HSFFIG because all the information about C structures it processes is
available. Anyway, even if you or somebody else implement this now, it
will most likely be available in 6.6 only, won't it?

Just for the future: from the original Template Haskell paper [2], I
got the impression that the ultimate goal was to be able to access the
whole tree of objects internally known to the compiler at the moment
when the TH constructs are involved, wasn't it? However I understand
that the priority is low.

Once I tried to implement (with some success) runtime introspection
for Hugs, but nobody seemed to show any interest...


[1] http://www.haskell.org//pipermail/template-haskell/2005-December/000501.html

[2] http://www.haskell.org/th/papers/meta-haskell.ps
Dimitry Golubovsky

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